1. Field of the Invention
The invention pertains generally to carburetors or single point air/fuel ratio controllers for internal combustion engines and is more particularly directed to an electronic injection carburetor having primary and secondary metering functions.
2. Description of the Prior Art
Generally, carburetors in the prior art are well known and comprise basically a metering jet or jets located within a venturi. The metering jets and appropriate slow and fast idle adjustments are sized to provide an approximate air/fuel ratio as fuel is drawn into the carburetor throat by the suction or pressure drop of the air flowing past the venturi restriction. Such carburetors further employ a reservoir and float mechanism controlling a fuel pump for normally providing an acceleration well to draw fuel from for rapid advancements of a throttle plate. An accelerator pump may also be provided and attached controllably to the throttle in such a case for rapid transient response.
These systems depend on mass air flow through the venturi to provide a pressure drop or vacuum indicative thereof for controlling the metering function and while efficient to some extent do not provide an exact air/fuel ratio for all operating conditions. For example, an idle setting at a desired or a slightly richer than desired air/fuel ratio with a fixed metering jet will produce an even richer air/fuel ratio at high speeds because the fuel drawn into the venturi increases at a rate faster than the air flow. This rich operation is not fuel efficient over a wide range of operating speeds where economy and not power is necessitated.
The sizing of the venturi in a conventional carburetor is a tradeoff between high and low speeds. It is necessary to manufacture the restriction small enough to supply enough metering vacuum at low speeds and just off idle but not so small as to restrict engine air flow at higher speeds. Further, with a venturi, distributional problems of fuel may arise. Normally the jets must be centered in the area of highest vacuum for correct vaporization of the fuel. The central air/fuel charge must then spread out through the manifold to reach the individual cylinders. A system that was capable of a more even initial distribution of the air/fuel mixture as the charge is formed in the carburetor throat would increase fuel distribution effectiveness.
Conventional single point carburetion systems are, however, relatively inexpensive and provide a good response for transient operations. They additionally provide the relatively rich air/fuel ratios needed for high speeds and power operation at wide open throttle positions.
An example of a mass air flow carburetor including a a venturi is shown in a U.S. Pat. No. 2,733,901.
Operating at precise air/fuel ratio is becoming more important as fuel expense rises because fuel efficiency and power output are functions of the correct air/fuel ratio. It is known that for very lean air/fuel ratios power is lost and driveability is sacrificed and for very rich air/fuel ratios fuel is wasted because not all the available fuel is utilized. A stoichiometric air/fuel ratio or one relatively close thereto is envisioned as a desirable operating point for many operating conditions.
Moreover, governmental regulation of the amount of emissions in the exhaust gas of an engine is becoming a design factor in carburetion systems. Air/fuel ratios exceedingly rich or lean will cause emission problems. For example, in a three way catalytic converter system for reducing exhaust emissions, it is known that the air/fuel ratio must be controlled within a fairly narrow window for correct operation.
Therefore, electronic fuel injection apparatus have been devised to meet the need for a more precise air/fuel ratio than a carburetor can provide. Advantageously, the apparatus utilize multiple solenoid injectors that are controlled electronically by a control unit that schedules the opening times and amount of fuel injected for each injector. The opening times of each injector valve are calculated by operating parameters of the engine such as speed, manifold absolute pressure, and others to precisely provide a desired air/fuel ratio.
These injection apparatus are to this date, however, relatively expensive and generally include an extra transient sensing circuit to respond quickly to changing engine needs. This is because open loop fuel schedulers normally respond to parameters that are changed by the transients instead of the variable that produce them. Further, some delays are built into the sensors supplying speed-density information such as a MAP sensor which must average all cylinder pressure changes into an overall signal.
Still further, additional circuitry in case of a system failure of fuel delivery is generally provided in an electronic fuel injection system to permit the engine to limp home once the loss of the basic fuel metering pulse is recognized. At that point, most of the control over the accuracy of the air/fuel ratio is sacrificed so the car may be driven to where service is available.
Thus, it would be advantageous to provide a system with the better features of both the speed-density fuel injection system and the mass air flow carburetor. This would reduce the cost of such apparatus below a fuel injection system but would provide a better control of air/fuel ratio than a regular carburetor. Such a system would further be more responsive to transient conditions and tolerant to system failures without additional circuitry and would alleviate distributional and other venturi problems.